Tin base alloys



United States Patent M 3,146,097 TIN BASE ALLOYS Lloyd R. Allen,Belmont, Mass, assignor to National Research Corporation, Cambridge,Mass, a corporation of Massachusetts No Drawing. Filed Apr. 23, 1962,Ser. No. 189,273 Claims. (Cl. 75-175) This invention relates to tin basealloys and more particularly to allotropic tin alloys which willdecrepitate to a fine powder at a predetermined time when subject tocertain environmental conditions.

Recently it has been proposed to place into orbit around the earth oneor more belts of thin microwave dipoles which will serve as anartificial scattering medium for radio signals in the centimeter band.It has been proposed that each dipole forming the belt of dipoles be ashort length of metallic wire that will reflect radio waves.

One of the important problems connected with achieving orbital dipolesis that of putting objects or materials into relatively permanent orbitswithout prior assurance that there will be no harmful side elfects.Accordingly, the lifetime of the dipoles forming the orbital beltsbecomes a matter of importance. For example, at altitudes higher than1000 kilometers atmospheric friction could not be depended upon to bringthe dipoles down and in general solar radiation pressure producesvariations in orbital eccentricity that are usually fairly small. It isknown, however, that such radiation pressure variations can be quitelarge within a narrow range of altitude and orbital inclination wherethey become resonant with the period of precession of the major axis ofthe orbit. Within this narrow range the periodic lowering of perigeemight well limit the lifetime to 1 or 2 years. Outside this range thebelt lifetime might be 1 or 2 decades.

It has been determined that one possible way of limiting the lifetime ofdipoles is to make the dipoles of white tin alloy which will undergotransformation to gray tin and disintegrate to powder under conditionsin space. Depending on the size of the powder formed, solar radiationpressure may bring the powder to earth or solar light may sweep thepowder into space.

The allotropic transformation of tin from white to gray tin has been thesubject of much research in the hundred or so years since its existencehas been known. The decrepitation to powder which results from [3 and atransformation and retransformation is due to a density change from 7.20(pl-phase) to 5.75 (ct-phase). The rate of transformation of white tin(/S-phase) to gray tin (ocphase) is known to be a function oftemperature. As White tin is lowered in temperature below thetransformation temperature, which is approximately 13 C., the rate oftransformation approaches a maximum rate which is close to about 35 to45 C., at which point the rate is many times faster than at the transustemperature of approximately 13 C. As the temperature of the tin isfurther lowered the rate of transformation starts to decrease. The rateof decrease has not been investigated at temperatures below about 120 C.

The transformation of white to gray tin is nearly always preceded by anindefinite incubation period which may run into years during which nosign of the transformation can be detected. While the transformation hasbeen studied in considerable detail the emphasis of such studies has bendirected towards means for preventing rather than 3,146,097 PatentedAug. 25, 1964 promoting the transformation. Accordingly, one of the mainproblems is that of providing allotropic tin alloys which promote thetransformation of tin within a predetermined time.

Accordingly, a principal object of the present invention is to providean allotropic tin alloy useful in producing dipoles which under orbitalconditions will maintain electrical integrity and physical size for apredetermined time and which will decrepitate under orbital conditionsin a predetermined time.

Another object of the invention is to provide tin base alloys which willundergo allotropic transformation and decrepitate to form a powder ofhigh area-to-mass ratio.

Another object of the present invention is to provide tin alloys which,when subject to electron or neutron radiation, require no induction ornucleation period for initiation of transformation.

A further object of the invention is to provide tin alloys which areradiation sensitive and which will decrepitate to a fine powder within apredetermined time upon thermal cycling about the transus temperature.

A still further object of the present invention is to provide tin basealloys which when isothermally exposed to cold temperatures willtransform from white to gray tin within a predetermined time.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the products possessing thefeatures, properties and the relation of components and the processesinvolving the several steps and the relation and order of one or more ofsuch steps with respect to each of the others which are exemplified inthe following detailed disclosure and the scope of the application ofwhich will be indicated in the claims.

For a fuller understanding of the nature and objects of the inventionreference should be had to the following detailed description.

The above objects are achieved and the problems indicated overcome bythe tin alloys of this invention which contain as essential ingredientsbetween 2 and 8 percent by weight of copper, about .05 to 4 percent byweight of germanium, the balance being tin.

In a more specific and more preferred embodiment the alloy compositioncontains from about 3 to 7 percent by weight of copper, about .1 to 3percent by weight of germanium, the balance being tin.

The alloys of the present invention can be prepared in accordance withconventional procedures and through recourse to known melting or castingtechniques. Thus the individual metals can be melt cast together and themelt allowed to cool and solidify. The melting operation can be carriedout, for example, in a resistance melting furnace, or by subjecting thecharge to induction heating or gas heating in a skull or crucible typeof container. Whatever the type of melting and casting employed, careshould be exercised to protect the molten metal from contamination. Thiscan be prevented, for example, by conducting operation under a vacuum ora non-reactive inert atmosphere such as argon, helium and the like orunder a flux blanket.

The individual metals used to form the alloys and charged to a meltingfurnace can be in any desired form such as powder shot, wire, sponge andthe like, and should be of the highest purity to insure production ofsatisfactorily pure alloy products.

In the present invention the alloys were prepared by introducing thedesired weights of the individual metals into a crucible in a vacuumresistance melting furnace. The charge of metals to the crucible wasthen heated and cast under vacuum. The alloys prepared in this mannerwere then tested for transformation under isothermal and thermal cyclingconditions by converting the cast alloy to foil or wire and then sealinga portion of the foil or wire in a glass tube. For exposure toisothermal temperatures the glass tube was immersed into a constanttemperature bath of the desired temperature. In the thermal cycling theglass tube was alternately dipped into baths held at the desiredtemperatures. When a specimen alloy was subjected to an irradiationfield strontium 90 was used as the source of radiation.

In accordance with the present invention it was determined that in orderfor a tin alloy to decrepitate to fine powder upon thermal cycling, itis necessary that the alloy be capable of retransforming from gray tinto a hard friable white state. Alloys which transform to hard white tinwill upon subsequent cooling convert to gray tin and decrepitate to formsmall particles. In contrast alloys which retransform from gray tin tosoft white tin will not decrepitate upon subsequent retransformation togray tin.

It has further been determined that the alloys of the present inventionare sensitive to electron and neutron radiation. For example, it hasbeen discovered that when the alloys of the present invention areirradiated with amounts as small as 10 rads the incubation period issufficiently reduced so as to be negligible. Thus the radiationsensitivity of the alloys of the present invention comprises one of theimportant features thereof. For example, when the present alloys areutilized as orbital dipoles, solar radiation will be sufficient toeliminate uncertain incubation times.

For a clearer understanding of the invention the following specificexamples are given. These examples are only illustrative and are not tobe considered as limiting the scope and underlying principles of theinvention.

Example 1 In this example an alloy containing 7 percent by weightcopper, /2 percent by weight germanium, the balance being tin, wasprepared. The tin alloy was then converted to foil and a portion of thefoil in the as rolled condition isothermally exposed to a temperature of80 C. After 96 hours of isothermal exposure the tin alloy hadtransformed 100 percent from white tin to gray tin. Thus, when the alloyis in a state of mechanical strain such as that resulting from coldrolling, the alloy does not require an incubation or nucleation periodto initiate transformation.

Exam plc 2 In this example an alloy containing 5% by weight copper, 3%by weight germanium, the balance being tin was prepared. The alloy wasconverted to foil. A portion of the alloy foil was inoculated with a fewgray tin crystals and exposed to cold temperatures. When isothermallyexposed to a temperature of-40 C. the tin alloy required 26 hours totransform from white tin to gray tin. When another portion of the tinfoil was isothermally exposed to a temperature of 80 C., 100% conversionto gray tin was completed in 120 hours. The gray tin when exposed to atemperature of +60 C. for 1500 hours transformed to the hard white phaseand decrepitated to powder upon subsequent exposure to 80" C. Anotherportion of the gray tin, when exposed to +70 C. for 100 hours,transformed to the hard white phase and decrepitated to powder uponsubsequent exposure to -80 C.

Example 3 In this example an alloy containing 3 percent by weight ofcopper, 3 percent by weight of germanium, the balance being tin, wasprepared. The alloy in the form of foil 4. was first irradiated with 1megarad of 1 mev. electron at 0 C. The alloy was then isothermallyexposed to temperatures of 6 C., 40" C., and C. The tin alloy foils werecompletely converted to gray tin in 96 hours at 60 C., 48 hours at 40C., and 48 hours at 80 C. The gray tin foil when heated to a temperatureof +40 C. for 100 hours transformed to hard white tin. When subsequentlyexposed to 80 C., the hard white tin converted to gray tin anddecrepitated to powder.

Example 4 In this example a 2 mil (.002 inch) diameter wire was madefrom an alloy similar to that of Example 3. The alloy wire wasirradiated with 1 megrad of 1 mev. electron and isothermally exposed toa temperature of 80 C. to convert the alloy wire to gray tin. The alloywire was then exposed to a temperature of 60 C. for about 36 hourswhereupon the wire retransformed from gray tin to hard brittle whitetin. Upon subsequent exposure to a temperature of 80 C. for about 24hours the wire transformed to gray tin and decrepitated to a mixture ofparticles varying in size from 1 mm. to powder. When the wire wasrecycled to 60 C. for 24 hours then to 80 C. for about 16 hours nopieces of the wire were larger than .1 mm. in any dimension. Continuedcycling increased the number of particles of 10 microns or less indiameter until after 4 cycles the wire had completely decrepitated toparticles of 10 microns or below.

Example 5 In this example a tin alloy contining 5 percent by weight ofcopper, 1 percent by weight of germanium, the balance being tin, wasformed and converted into a 2 mil wire. A small section of the wire(approximately 3 mm.) was then transformed to the gray tin phase byirradiation and isothermal exposure at -80 C. Exposure of the gray tinphase to a temperature of 60 C. for 2200 hours caused reversion to thehard brittle white phase which decrepitated to powder upon its reversionto the gray phase at -80" C. Three cycles between +60 C. and 80 C.completely converted the wire to a powder having average particle sizeof less than about l020 microns.

Example 6 In this example a tin alloy containing 5 percent by weight ofcopper, 2 percent by weight of germanium, the balance being tin, wasformed and converted into a 2 mil wire. Similar to Example 5 a smallsection of the wire was then transformed to the gray tin phase byirradiation and isothermal exposure at 80 C. Exposure of the gray tinphase to a temperature of 60 C. for 1000 hours caused reversion to thehard brittle white phase which decrepitated to powder upon itsreconversion to the gray phase at 80 C. Five cycles between +60 C. and40 C. completely converted the wire to a powder having an averageparticle size less than about 10 microns.

Since certain changes may be made in the above products and processwithout departing from the scope of the invention herein involved, it isintended that all matter contained in the above description shall beinterpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An allotropic tin base alloy consisting essentially of about 2 to 8percent by weight of copper, about .05 to 4 percnt by weight ofgermanium, the balance being essentially tin.

2. An allotropic tin base alloy consisting essentially of about 3 to 7percent by weight of copper, about .1 to 3 percent by weight ofgermanium, the balance being essentially tin.

3. An allotropic alloy consisting essentially of about 5 percent byweight of copper about 1 percent by weight of germanium, the balancebeing essentially tin.

4. An allotropic alloy consisting essentially of about 5 percent byweight of copper, about 2 percent by weight germanium, the balance beingessentially tin.

5. An allotropic alloy consisting essentially of about 5 pecent byweight of copper, abouut 3 percent by weight 6 OTHER REFERENCES Hansen:Constitution of Binary Alloys, McGraw-Hill Book Company, Inc., New York,1958, pp. 633-636.

Jaffee et al.: Technology of Germanium, Transactions germanium thebalance being essentially 5 American Electrochemical Society, vol. 89(1946), page 287. References Cited m the file of thls patent Guerter etal.: The Systems Tin-Germanium and Tin- UNITED STATES PATENTS Beryllium,Technical Publications of the International Tin 1,869,378 Konigsberg Au2, 1932 Research and Development Council, Series A, Number 2,745,046Lark-Horovitz et a1. May 8, 1956 1 1 p

1. AN ALLOTROPIC TIN BASE ALLOY CONSISTING ESSENTIALLY OF ABOUT 2 TO 8PERCENT BY WEIGHT OF COPPER, ABOUT .05 TO 4 PERCENT BY WEIGHT OFGERMANIUM, THE BALANCE BEING ESSENTIALLY TIN.